Why are ruminal cellulolytic bacteria unable to digest cellulose at low pH?

Ruminant animals depend on cellulolytic ruminal bacteria to digest cellulose, but these bacteria cannot resist the low ruminal pH that modern feeding practices can create. Because the cellulolytic bacteria cannot grow on cellobiose at low pH, pH sensitivity is a general aspect of growth and not just a limitation of the cellulases per se. Acid-resistant ruminal bacteria have evolved the capacity to let their intracellular pH decrease, maintain a small pH gradient across the cell membrane, and prevent an intracellular accumulation of VFA anions. Cellulolytic bacteria cannot grow with a low intracellular pH, and an increase in pH gradient leads to anion toxicity. Prevotella ruminicola cannot digest native cellulose, but it grows at low pH and degrades the cellulose derivative, carboxymethylcellulose. The Prevotella ruminicola carboxymethylcellulase cannot bind to cellulose, but a recombinant enzyme having the Prevotella ruminicola catalytic domain and a binding domain from Thermomonspora fusca was able to bind and had cellulase activity that was at least 10-fold higher. Based on these results, gene reconstruction offers a means of converting Prevotella ruminicola into a ruminal bacterium that can digest cellulose at low pH.

[Effect of type of starch and quantity in rations on apparent and true digestibility of nitrogen and nitrogen balance in sheep]

In a digestibility trial 5 semisynthetic rations were fed in 3 periods to 10 male sheep to examine the effects on N balance, components of faecal nitrogen and N digestibility. The rations contained constant amounts of nitrogen but different contents of cellulose and two different types of starch (untreated and steamflaked). Content and type of starch did not show any noticeable effect neither on excretion of undigested dietary nitrogen nor on true digestibility. There could not be noticed any effects on the apparent N digestibility by changing contents of cellulose or untreated starch. If the rations contained steamflaked corn starch, the animals excreted more faecal nitrogen and therefore showed a lower apparent N digestibility. Especially the water soluble N fraction of the faecal nitrogen was clearly higher. Compensation took place through a lower N excretion with the urine. The reason for increased faecal N excretion may be higher microbial protein synthesis in the rumen. This was accented by the allantoin excretion with the urine.

SOME EFFECTS OF RUMINAL INFUSIONS OF UREA AND UREA-SUCROSE ON UTILIZATION OF OAT STRAW BY COWS

Four diets, straw, straw plus urea, straw plus urea and sucrose, and straw plus urea but with a different mineral supplement than fed with diets 1 to 3, were compared using rumen-fistulated cows. Urea or urea and sucrose were dissolved in water and administered as continuous ruminal infusions. Increases of about 29% in dry matter digestibility and 17% cellulose digestibility were obtained with urea but not with urea-sucrose. Bacterial protein in rumen fluid was increased by an average of 63% by both urea and urea-sucrose treatments. Rumen ammonia and blood urea levels were increased tenfold and threefold, respectively, by the urea infusion, but the response to urea-sucrose was only one-half as great. Sucrose appeared to be used as an energy source in preference to the straw, as indicated by the increased rate of urea utilization, but tended to depress intake and digestibility of straw as compared with urea only. Dry matter intake, nitrogen balance and total short-chain fatty acids were not significantly altered by the treatments, but all snowed response trends. Effects of dietary treatments on proportions of short-chain fatty acids in the rumen were not consistent.